Transient Numerical Model on the Design Optimization of the Adiabatic Section Length for the Pulsating Heat Pipe

In the application of pulsating heat pipes (PHPs), the lengths of the adiabatic sections are usually determined by the distance between the heat source and the heat sink, and have important effects on the performance of PHPs. However, there was little research on the effect of the adiabatic section lengths on the performance of PHPs. In this work, a new transient numerical model was proposed to investigate the transient flow and the heat transfer for PHPs with various adiabatic section lengths of 60, 120, 180, and 240 mm. Based on the numerical results, the flow and the heat transfer characteristics of the PHPs were analyzed. It was found that the flow velocities in the PHP with different adiabatic lengths increased with the increase in the heat input, and the mean velocity was calculated to be in the range of 0.139–0.428 m/s, which was consistent with the previous experimental results. The start-up performance of the PHP was better with shorter adiabatic section length. Furthermore, the thermal resistances of the PHPs with different adiabatic section lengths were calculated to analyze the effects of the adiabatic section length on the performance of the PHP. The results showed that when the heat input was 20 W, the PHP with the adiabatic section of 60 mm showed the lowest thermal resistance, whereas the PHP with longer adiabatic section length presented lower thermal resistance at high heat input (≥25 W).

Experimental Study on the Starting-Up and Heat Transfer Characteristics of a Pulsating Heat Pipe under Local Low-Frequency Vibrations

Vibrations have attracted much attention as an effective method for enhancing heat transfer in pulsating heat pipes (PHPs). This study mainly investigates and explores the effects of local low-frequency vibrations on the starting-up and heat transfer characteristics of a PHP. The starting-up temperature and average temperatures along the evaporation section of the pulsating heat pipe were experimentally scrutinized, along with thermal performance, under local vibrations on evaporation, condensation and adiabatic sections, respectively. The following important conclusions can be derived by the experimental study: (1) The effect of vibrations at the evaporation section and at the adiabatic section during the starting-up time of the PHP were more significant than that at the condensation section; (2) vibrations at different positions could reduce the starting-up temperature of the PHP—the effect of the vibrations at the evaporation section was the best when heat power was lower, while the effect of vibrations on the adiabatic section was the best when heat power was higher; (3) vibrations at the evaporation and adiabatic sections could reduce the thermal resistance of the PHP, but vibrations at the condensation section had little effect on the thermal resistance of the PHP; (4) vibrations at the evaporation and adiabatic sections could effectively reduce the temperature at the evaporation section of the PHP, but the vibrations at the condensation section had no effect on the temperature at the evaporation section of the PHP. This paper shows that local low-frequency vibrations have positive effects on the heat transfer performances of PHPs.

Experimental Study on the Starting-Up and Heat Transfer Characteristics of Pulsating Heat Pipe Under Local Low-Frequency Vibrations

This study mainly experimentally investigates and explores the effects of local low-frequency vibrations on the starting-up and heat transfer characteristics of the pulsating heat pipe. A micro motors with the vibration frequency of 200 Hz were imposed on the external surface of evaporation, condensation and adiabatic section of the pulsating heat pipe, respectively, and the starting-up temperature and the average temperatures along the evaporation section as well as the thermal performances of the vibrating heat pipe were experimentally scrutinized under the local vibrations of different positions. The following important conclusions can be achieved by the experimental study: 1) The effect of vibrations at the evaporation section and at the adiabatic section on the starting-up time of pulsating heat pipe is more significant than that at the condensation section. 2) The vibrations at different positions can reduce the starting-up temperature of the pulsating heat pipe. The effect of the vibrations at the evaporation section is the best as the heating power is lower, and the effect of the vibration at the adiabatic section is the best as the heating power is higher. 3) The vibrations at the evaporation section and at the adiabatic section can reduce the thermal resistance of the pulsating heat pipe. However, the vibrations at the condensation section have little effect on the thermal resistance of the pulsating heat pipe. 4) The vibrations at the evaporation section and at the adiabatic section can effectively reduce the temperature of evaporation section of the pulsating heat pipe, but the vibrations at the condensation section have no effect on the temperature of evaporation section of the pulsating heat pipe.

Influence of Geometrical Changes in an Adiabatic Portion on the Heat Transfer Performance of a Two-Phase Closed Thermosiphon System

In this study, a modified non-uniform adiabatic section in a Two-Phase Closed Thermosiphon (TPCT) is proposed where the uniform section was replaced by convergent and divergent (C-D) sections. The heat transfer analysis was performed on the modified TPCT and their findings were compared with standard TPCT. The deionized water (DI) in the proportion of 30 vol% is filled in both the TPCTs. Further, the heat transfer performance analysis was carried out for three different orientations, such as 0°, 45° and 90°, and heat input was varied from 50 to 250 W. The effect of these geometrical changes and inclination angles on the heat transfer performance of both the TPCT were evaluated to compare the thermal resistance, wall temperature variation and heat transfer coefficient. The non-dimensional numbers such as Weber (WE), Bond (BO), Condensation (CO) and Kutateladze (KU) were investigated based on heat fluxes for both TPCTs. By introducing the convergent-divergent section nearer to the condenser, the pressure before and after the C-D section was increased and decreased. This enhances the heat transfer in the evaporator slightly up to 2% and 1.4% at horizontal and 45° orientation, respectively, in Non-Uniformed Adiabatic Section (NUAS) TPCT when compared to Uniformed Adiabatic Section (UAS) TPCT. The thermal resistance of NUAS TPCT was reduced by up to 4.5% relative to UAS TPCT in horizontal and 45°. The results of the non-dimensional number also confirmed that NUAS TPCT provided better performance by enhancing 2% more pool boiling characteristics, interaction forces and condensate returns. Several factors such as gravity assistance, fluid accumulation, pressure drop and thermal resistance exert an influence on the heat transfer performance of the proposed NUAS TPCT at various orientation angles. However, different type of cross-sectional variations subjected to orientation changes may also get influenced by several other parameters that in turn affect the heat transfer performance distinctly.

Thermal Performance of Thermosyphons Intended for Evacuated Tube Solar Collector Using Graphene Oxide Nanofluids

Vacuum tube solar collectors are composed by two concentric glass tubes with the annular space evacuated. At the inner tube a thermosyphon is placed inside a metallic fin in order to absorb sun’s irradiation and heat running water placed at a manifold. Thermosyphons are passive heat transfer devices that absorb heat at the evaporator region, evaporating the working fluid that reaches the condenser in the form of steam. At the condenser, heat is dissipated to the environment, condensing the working fluid that returns to the evaporator, closing the thermodynamic cycle. In this study, thermosyphons with three different working fluids (5 and 10% graphene oxide nanofluids and distilled water) were built and experimentally tested. The evaporator and the adiabatic section have an outer diameter of 8.33mm and lengths of 1,600mm and 40mm, respectively. The condenser has an outer diameter of 13.40mm and a length of 35mm. The filling ratio used was 50% of the evaporator’s volume. A resistive tape wrapped at the evaporator and connected to a power supply was responsible for heating the working fluid by Joule effect, and water flow rates of 0.50, 0.75, and 1.00L/min were responsible for condensing the working fluid at the condenser. Heat loads of 35, 55, and 75W were applied to the devices and K-type thermocouples were responsible for acquiring temperature data from the thermosyphons, allowing the thermal analysis based in the temperature distribution and thermal resistance for each working fluid. The best working fluid for the conditions proposed, out of the three investigated, was 5% graphene oxide.

Fabrication and Characterization of Heat Pipe with Composite Structure for the Adiabatic Section

Background: Innovative cooling technology is required in every field of life ranging from satellite to terrestrial applications. Novel heat rejection system is of great concern for space applications. Futuristic applications in heat pipe will involve composite structures in various ways as they offer flexibility in design with their inherent advantage of being light weight. However it remains a challenge to join a composite structure with metals. This study investigates the effect of composite section in the adiabatic region of the heat pipe and also proposes a novel approach to joining metals with composite structure. Methods: A flexible composite tube such as carbon fiber reinforced thermoplastics makes the adiabatic section. This section is adhesively bonded with the metal tubes i.e. evaporator and condenser section. Inherent roughness of the metal tube makes first layer for mechanical interlocking followed by adhesive bonding. Results: The effect of adiabatic, condenser and evaporator length, for a normal vs. composite heat pipe, on specific thermal conductivity is evaluated. Conclusion: The numerical studies confirm that the use of composite material for adiabatic section improves performance of heat pipe. It is proposed to use reinforced thermoplastic as the material for adiabatic section.